Sort by updated time

My feeds weren't actually displayed in order according to their updated time (for either "sort by newest" or "sort by oldest"). By adding some debug statements, I saw that the API was not returning them in any sorted order, despite being called with "date_reverse" or "feed_dates" as it expects.

So instead of debugging what's wrong with TT-RSS, writing a patch for it, and getting that approved in the main project, this workaround makes the mobile version sort it client-side.

(Sorry for the slightly messy branch history with the reverted commit from a different pull request.)